In recent years, with an increasing demand for safety and security, monitoring cameras have been installed in a public space such as a station or an airport, and important facilities. In the related art, an observer monitors a monitoring camera all the time. Therefore, there is an attempt to reduce efforts for monitoring and improve monitoring efficiency using image recognition in order to prevent the observer from overlooking the monitoring cameras due to an increase in the number of monitoring cameras and the fatigue of the observer.
Patent Document 1 and Patent Document 2 disclose a technique for counting the number of persons in a monitoring place. Patent Document 1 discloses a technique that extracts the background using a background difference and counts the number of persons who move across a monitoring area which is orthogonal to a person's passage. A plurality of monitoring areas is prepared, and a variation in count value between the monitoring areas is used to accurately count the persons even though disturbance occurs.
Patent Document 2 discloses a technique in which a camera is provided above a passage such that the optical axis is aligned with the vertical direction, a motion vector is extracted at a boundary line provided on the image in order to count the number of persons, and a vertical component of the motion vector with respect to the boundary line is integrated to count the number of persons passing through the passage.
Patent Document 3 discloses a technique that extracts features corresponding to the number of persons from an image without counting the number of persons to calculate the degree of congestion. In Patent Document 3, the number of changes in each pixel or each local area for a predetermined amount of time is calculated and the degree of congestion is calculated on the basis of the number of changes on the assumption that, “when there are a large number of passengers, the number of changes between images captured at different times is increased”.
In addition, in the related art, various techniques have been proposed which estimate the degree of congestion of persons in an image. For example, a technique has been proposed which calculates motion vectors, calculates the integral value of the motion vectors, and counts the number of persons in unit of the integral value (for example, see Patent Document 4).
A technique has been proposed which detects the heads of the persons, measures the number of heads, and estimates the degree of congestion (for example, see Patent Document 2).
A technique has been proposed which estimates the degree of congestion using the area of the background extracted by an inter-frame difference or background difference process (for example, see Patent Document 3 or Patent Document 5).    Patent Document 1: JP-A-2002-074371    Patent Document 2: JP-A-2005-135339    Patent Document 3: JP-A-2004-102380    Patent Document 4: JP-A-2005-128619    Patent Document 5: JP-A-11-282999